This is a study of the error rates of mutator, anti-mutator and wild type T4 DNA polymerase (gene 43) in 2-aminopurine-substituted DNA in vivo. Base analogue-induced mutation rates can be influenced both by the degree of incorporation of the analogue into the DNA and by the frequency of errors of replication past incorporated 2-aminopurine. An appropriate method is being sought to separate the incorporation from the replication step of mutegenesis by 2-aminopurine. Preliminary evidence suggests that separation of incorporation and replication errors may be achieved by mixed infection with one phage, substituted with 2-aminopurine and carrying one allele of gene 43, and a second, unsubstituted phage which supplies a second allele (mutator, anti-mutator or wild type) of gene 43. Quantitative analysis of mutagenesis in these mixed infections may allow determination of error rates in vivo for different gene 43 alleles independent of incorporation rates.